The purpose of this research will be to discover if the capacity to repair damaged tissues and adapt to a toxic environment (NO2) is less in the lungs of old rats than in young and adult rats. Specifically, we will determine if the amount of tissue damage is more or less in old rats by means of light and electron microscopy. By labeling dividing cells with tritiated thymidine, we will show whether repair of the tissue damage is delayed, and whether the cells adapt to the NO2 after repair. In the peripheral alveoli, we will determine if the increase in cell renewal normally seen in young animals during exposure occurs also in older animals. These results will demonstrate if the low cell renewal rate reported in normal old mice increases during stress with the same efficiency as is seen in younger animals.